Particular problems have been encountered and particular needs have arisen in producing oil from wells characterized by low flow rates and downhole pressures. Such wells are commonly referred to in the oilfield as "stripper wells," and may produce at rates as low as one or two barrels per day, or less. The conventional method for producing oil from these wells is by the use of surface-mounted pumpjacks that are connected to downhole mechanical rod pumps by strings of sucker rod. The entire string of sucker rod, which extends downward from the surface to the producing formation, is reciprocated several times per minute by the pump jack, which is in turn driven by a motor. In stripper wells, the pump jack is usually operated intermittently on a preset pumping cycle controlled by a time clock.
Unfortunately, the capital outlay required to install a pump jack, drive motor, downhole pump and sucker rod is considerable, and is hard to justify where the potential production is minimal. Furthermore, the operation and maintenance costs for pumpjack installations are expensive. The chances for mechanical failure are great because of the reciprocating action of the pump, which must be transmitted downhole via the string of sucker rod. Whenever maintenance is required, a workover rig is needed in order to pull the string and pump from the hole. In sandy wells, the maintenance problem is particularly severe due to the abrasive action of the sand particles during the reciprocating motion of the pump. Pump cavitation can also be a problem when the gas to liquid ratio is high.
The use of jet pumps for producing liquid and entrained gaseous hydrocarbons from higher volume wells is also well known. Conventional petroleum recovery pumping systems employing jet pumps are commercially available from oil field supply companies such as National Supply Company, the Guiberson Division of Dresser Industries, and the Kobe Division of Baker International Corporation. Patents disclosing the use and operation of jet pumps include, for example, the following U.S. Pat. Nos.: 4,310,288; 2,285,638; 4,135,861; 2,080,623; and 2,041,803.
Generally speaking, jet pumps contain few or no moving parts. A power fluid, usually water or oil, is supplied at high pressure to the nozzle of the jet pump, converting the pressure head into a high velocity jet. Pumping action begins when the fluid in the production inlet chamber is entrained by the jet stream emerging from the nozzle. In the throat, the produced fluid acquires high velocity from the power fluid. This velocity head is subsequently reconverted in a diffuser section to a pressure head having a pressure that is adequate to move the fluid to the surface. Historically, petroleum recovery pumping systems utilizing jet pumps have been primarily recommended for applications involving abrasive or corrosive fluids, applications where the production capacity exceeds that of most reciprocating pumps, and applications where the gas-liquid ratio exceeds the limits for reciprocating pumps.
Although the various commercially available jet pumps utilize differing nozzle and internal flow configurations, they share certain draw-backs and disadvantages which have limited their usefulness for many applications. One principal disadvantage has been that conventional jet pumps have not been effective for wells producing at rates less than about 200 barrels per day. Also, the cost of the conventional, commercially available jet pump installations has heretofore greatly exceeded that which can be justified with low volume wells.
Another disadvantage is that such pumps have required a level of suction fluid within the well that is, for example, about 25% of the distance from the pump location to the surface. Still another disadvantage is that such pumps have typically required a relatively high bottom hole pressure in order to function properly. Because of this, it has heretofore been impossible to take a well to depletion with a jet pump unless the well has an active water drive to maintain the bottomhole pressure at high level.
Thus, although jet pumps are more energy efficient and less susceptible to breakdown and wear than conventional mechanical pumps, their usefulness has been limited by the foregoing disadvantages. A petroleum recovery pumping system is therefore needed that can be effectively and economically operated through use of a jet pump in oil wells having production rates of less than about 100 barrels per day, and as low as one or two barrels a day, with little or no bottomhole pressure.